1. Technical Field
The present invention relates in general to an improved wireless communication system and in particular, the present invention relates to an improved CDMA cellular telephone system. Still more particularly, the present invention relates to a method which allows improved sector handoff performance in highly sectorized CDMA cells in the vicinity of a base station antenna.
2. Description of the Related Art
Mobile radio telephones and mobile telephone systems are well known in the prior art. A mobile telephone system generally includes a mobile hand held radio telephone transceiver, and a base station connected to a local telephone switching system by a landline. Typically, a cellular network has an assigned set of landline telephone numbers that allows users of a mobile hand held transceiver to place and receive calls within a limited range of the base station's antenna.
Mobile hand held transceivers which are specifically designed for telephone communication are often called cellular telephones or mobile radio telephones. Cellular telephone systems have developed rapidly since the early 1980s. Persons equipped with small mobile communication devices, such as mobile radio telephone, can utilize a cellular radio system to communicate in the same way as a hard wired household telephone which utilizes landline carriers.
Due to the increase in cellular telephone utilization, digital communication is gaining popularity over analog communication. Digital communication topologies can simultaneously support many more users than analog topologies in a given frequency spectrum. Since a limited number of frequencies and channels are available, analog systems can only support a very limited number of simultaneous users. A digital radio system can handle more than 20 times the capacity of a traditional analog radio system in the same frequency spectrum. Digital systems employ methods where multiple users share the same frequency. This concept is commonly referred to as "spread spectrum communication." Distinct digital channel sharing topologies have emerged, such as code division multiple access (CDMA), global system for mobile communication (GSM) and time division multiple access (TDMA). A digital system has a considerably larger data transmission capacity than an analog system and higher capacity translates to higher revenues for cellular system owners.
Typically, a mobile radio telephone system assigns a fixed base transceiver to geographic areas. In a typical cellular system, a geographical area is divided into small areas, called cells. Coverage is typically measured as a radius from the base station antenna. Each cell has a predefined coverage radius, for example, large cells commonly referred to as "mega cells" can have a coverage of over 20 kilometers (13 miles). Additionally, macro cells have a coverage from 1 to 20 kilometers, micro cells have a coverage of approximately 1 kilometer, while pico cells have a coverage of only 100 meters. Each cell has its own radio transceiver commonly referred to as a base station. If necessary, each cell can be further subdivided into smaller cells through cell splitting and/or sectorization by steering antenna patterns.
In a typical CDMA system, a honeycomb type pattern of cells is created which utilizes the same range of radio frequencies. In many respects, CDMA is superior to TDMA and Frequency Division Multiple Access (FDMA) because CDMA systems can utilize precisely the same frequency spectrum in all sectors without significant interference among sectors. In CDMA the same set of frequencies can also be utilized from cell site to cell site. A CDMA topology assigns a different binary sequence or code to a transmitted signal to identify the message for an individual mobile transceiver. This allows a single frequency to serve multiple users.
The specifications for CDMA operation are outline in the Electronic Industries Association/Telecommunications Industry Association (TIA/EIA) IS-95-A & TSB74 standards document entitled Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System or CDMA Principles of Spread Spectrum Communication, by Andrew J. Viterbi.
The correlator, a subcircuit within the mobile transceiver, accepts only energy from identified binary sequences or codes and de-spreads across the spectrum. The mobile receiver correlates its input with the desired carrier and receives the appropriate data. Received signals having codes which do not match the receiver codes are not de-spread in bandwidth and contribute only to noise. The signal to noise ratio of the desired signal is enhanced at the detector of the mobile transceiver by a factor known as the processing gain. One advantage to a CDMA system is that the receiver is not sensitive to worst case interference, but to the average interference.
CDMA has often been dismissed as unworkable in the mobile radio environment because of signal strength differential, as some users are located near the base station and others are located far away. To accommodate the far away users, a spreading bandwidth must be thousands of times greater than the data rate, making the spectral efficiency intolerable. If a reasonable bandwidth is chosen the signal cannot be received from distant users because the users near the base station significantly interfere. To overcome this inefficiency, the transmitter of each mobile is controlled such that the received powers from all users is roughly equal to achieve an interference averaging concept of power control. A similar process is performed on signals sent to the mobles from the base station(s).
Computerized switching is essential to the operation of cellular radio communication. When a communicating mobile transceiver is switched from one cell to another, a transfer of channels must take place without interruption, or at most a brief delay. The growth of electronic switching systems and the development of microprocessors have made seamless communication possible within areas covered by cellular providers. The U.S. Federal Communications Commission (FCC) continues to allocate and license additional radio frequencies. Due to increasing popularity of cellular telephones, in recent years the FCC has awarded additional frequency bands to be utilized by cellular telephone technology.
A cellular telephone system typically includes cellular subscriber units, which are portable, and cellular base stations, which are connected to the public telephone company via one or more cellular switching networks. Each cellular subscriber has an assigned cellular telephone number that allows the user to place and receive calls within a widespread range of the cellular base stations, such as throughout a metropolitan area.
Cellular telephone systems are thus based on a structure of associated cells. Cells are specified geographic areas that are defined for a specific mobile communication system where each cell has its own base station(s) and controllers) interconnected with a public telephone network.
Communication between base stations and mobile subscribers is established by negotiation protocols upon call origination. As a user passes from cell to cell, the cellular service allows calls in progress to be handed over without interruption (soft handoff) or minimal interruption (hard handoff) to adjacent cells thus providing seamless communication.
Handoffs of a mobile radio telephone between cells and sectors in CDMA ideally occur as soft handoffs. In a CDMA system, mobile radio telephone stores a list of active channels (utilized for demodulation purposes) which are being received at acceptable levels in an "active set". The active set members are sector and/or cell channels that transmit and receive identical information with the mobile. A soft handoff occurs when the active set contains more than one sector and/or cell. When a communication path becomes weak, the mobile radio telephone will remove the weak channel from the active list, via protocols defined in IS-95, but there is no noticeable disruption in the communication link. Similarly, if a new sector and/or cell increases in strength, the mobile and network will add this new sector and/or cell to the active set of the mobile via IS-95 protocols which are comprised of messages and thresholds, etc.
A hard handoff typically occurs when communication on a particular frequency is dropped and a new channel having a different frequency is acquired. For example, a termination of communication on one frequency and initiation of a new communication link on another frequency is a hard handoff. Hard handoffs typically occur when a mobile moves outside a coverage area and the call is switched from one service provider to another who utilizes a different frequency. In a CDMA system, typically, all cells are owned by a single service provider and operate in the same frequency spectrum. Therefore, soft handoffs are the prevalent method of handoffs. Hard handoffs can occur when adjacent cells are utilizing the same frequency. Typically, hard handoffs are less common than soft handoffs in CDMA and a hard handoffs occur according to the service providers system parameters and network functionality.
A typical six sectored CDMA cell contains a twelve element antenna. Each antenna element can provide a highly directional radiation and reception pattern. As depicted in FIG. 1, a single element radiation and reception pattern projecting from antenna mast 6 is commonly referred to as a sector. A twelve element antenna provides six sectors in a radial configuration placed adjacent to one another separated by imaginary partition lines 5. Six patterns each rotated 60 degrees encircle antenna mast 6 and provide coverage within cell 34. As the radiation patterns are offset by 60 degrees, side lobes 4 also orient in 60 degree offsets. Six of the twelve elements are typically utilized for transmission and reception, while the other six are utilized for reception only, providing dual antenna reception diversity. Therefore, in each sector, one antenna is utilized for transmitting and receiving and the other antenna is utilized for receiving only. Radially configuring main lobe 2 in each 60 degree sector creates significant side lobe interference in the area depicted by side lobe area 9.
High capacity cells can employ 8 or 10 sectors for improved performance. An ideal antenna pattern has a main lobe 2 which provides clear sectorization at a far distance from antenna mast 6, however, near antenna mast 6 there is significant overlap of radiation energy due to minor lobes called side lobes 4 and back lobes 8. Side lobes 4 and back lobes 8 cause interference to other antennae in the array in side lobe area 9. Additionally, each antenna transmitting element also has vertical profiles, hence, vertical side lobes and back lobes. Therefore, within side lobe area 9 there is no single sector providing dominant coverage and all sectors provide weak coverage.
As sectorization within cells increases, the probability of dropping a call near and around antenna mast 6 also increases. This is due to the attributes of the antenna array's reception and radiation pattern in the vicinity of the antenna. An antenna array inherently has undesirable interference as a result of overlapping side lobes of individual antenna elements near antenna mast 6. Each sector receives a significant amount of interference due to leakages, side lobe radiation and back lobe radiation due to the inherent electromagnetic characteristics near the antenna. Side lobes are most prevalent near antenna mast 6 and signal quality can be severely effected in this region. In a CDMA system, all sectors of an antenna may be received by the mobile at approximately the same strength near or under antenna mast 6.
Each sector produces a continuously broadcasted pilot signal so any mobile radio telephone scanning the spectrum for usable pilots can decide which sector will provide the best communication link. When moving towards an antenna mast, the change from one dominant pilot to a number of weak pilots of equal strength can happen very suddenly. This is due to the lack of dominant coverage within side lobe area 9. Less than desirable coverage of all sectors occurs due to side and back lobes overlap. Alternatively, a change from many weak pilots to a, yet unknown dominant pilot can also occur as the mobile radio telephone moves away from antenna mast 6.
Sudden changes in reception due to a change from a single strong pilot to multiple weak pilots can degrade the forward link to such an extent that is not possible for the network to instruct the mobile radio telephone to add the newly acquired weak pilots, hence, the lone communication link may not support the mobile radio telephone. Without newly acquired pilots, eventually the call will drop or the frame error rates will increase, impacting system capacity and performance and the user may be disconnected.
Additionally, as more sectors per cell are implemented, the Pilot Carrier-to-Interference (C/I) ratio of each of the sectors near and around antenna mast 6 are relatively equivalent. Pilot C/I ratios are low around the base of antenna mast 6 and therefore there is no dominant sector to be identified by a mobile radio telephone. Pilot C/I ratios at the base of antenna mast 6 are often lower than the threshold level utilized by a mobile radio telephone to add carriers and initiate a communication link. In other words, the pilots from adjacent sectors may not be acquired by a mobile radio telephone near the base of antenna mast 6 because received signals are below specified threshold levels and a call in progress can drop or degrade drastically without identification of potential alternate paths for diversity reception and demodulation.
Currently, sectored cells function at about 80% forward link loading during peak time. Alternately described, each sector on the average utilizes 80% of its total high power amplifier power to service users. With typical 16% pilot power, the pilot C/I for each sector at or around the antenna mast is approximately -14.8 dB. Therefore, in a six sector cell the mobile radio telephone may be moving in one of the sectors (its pilot C/I about -7 dB when dominant) toward the base station antenna and suddenly, the current sector and all other sectors drop to -14.8 dB. Since, the add pilot threshold is typically -14.0 dB, a handoff transaction can not occur because the additional sectors do not trigger handoff and consequently the additional sectors are not acquired by the mobile radio telephone. A single sector reception having a pilot of -14.8 dB is insufficient to sustain adequate communication and a call may drop or significantly degrade under these circumstances. This impacts call quality and also cell capacity as the single sector must transmit at very high power in attempting to maintain the call with the mobile.
At lower loading, for example 60% loading, the pilot C/I of all the sectors are approximately -13.5 dB, hence a message to add communication links to the mobile radio telephone would be sent by the mobile radio to the base station. However, a sharp decline of signal strength at the base of antenna mast 6 causes protocol problems. For example, the movement of a mobile radio telephone producing a change from a single pilot C/I ratio from -5.7 dB to all pilots at -13.5 dB may also cause a call to drop. As soon as the mobile radio telephone detects that pilots exist which are all above the add pilot threshold, the mobile attempts to send an add signal to the network, via the first original sector. The communication instructing the mobile radio telephone to add other sectors may not be received by the mobile radio telephone because of the significant interference from the other sectors. The add message is re-attempted a number of times (IS-95 protocol) but after a number of attempts the system drops the call. Otherwise, the add sector message may get through on subsequent re-sends, but the frame error rate and transmit power requirements of the base station will increase impacting, capacity, and performance until additional sectors are acquired.
At present, the only solution available is to data-fill the add pilot threshold and drop pilot thresholds to -16.0 and -18.0 dB respectively. However, this extends the cells handoff boundary outwards as the mobile radio telephone moves away from a base antenna to other cells. Changing pilot thresholds redefine cell boundaries and creates cell to cell handoff problems. Lower handoff thresholds compromises resource capacity, such as channel elements and RF capacity as additional sectors in handoff with a mobile radio telephone must transmit to the mobile radio telephone although their signal is never utilized for demodulation purposes.
The disadvantage of lowering thresholds is that excessive handoff may result, which compromises capacity. Additional software development has been attempted to improve handoff performance, however, this is costly. It should therefore be apparent that an improved method for handoff performance utilizing CDMA technology in highly sectorized cells in the vicinity of the base antenna is highly desirable.